1. Field of the Invention
The present invention relates to production of can bodies, and, more particularly, to a method of automatic electric seam resistance welding of a thin-walled can body and an apparatus for carrying out the method.
2. Description of the Prior Art
Generally, a method of automatically producing a can body by using an electric seam resistance welding method comprises the steps of deforming a thin rectangular flat metal blank into a cylindrical can body blank and welding the side ends of the cylindrical blank to form a longitudinal seam. Generally the metal blank has a rectangular shape after being cut from a metal sheet by a slitter, and is deformed into the cylindrical blank by a roll-forming machine. Both side ends of the cylindrical blank are inserted into a pair of corresponding guide grooves of a guide rail. Keeping the side ends inside the guide grooves, the cylindrical blank is conveyed along the guide rail into a roller-electrode station of an electric seam resistance welding apparatus by a suitable conveying means, e.g., a chain circulating in parallel with the center axis of the cylindrical blank and provided with hooks. Then, the side ends overlap each other and are welded between the roller electrodes in a longitudinal direction, whereby a welded can body is obtained.
The guide rail (a so-called Z-shape rail) and the roller electrodes are well-known by persons having ordinary skill in the art and are described in, for example, Japanese Utility Model Publication No. 49-26817 and Japanese Patent Publication No. 54-26213 (corresponding to U.S. Pat. No. 4,160,892). When the seam welding is carried out, portions of the overlapped ends of the cylindrical blank are fused and pressed by the roller electrodes, so that a rotative distortion is generated so as to open the overlapped ends. Taking the rotative distortion into consideration, the depth of the guide grooves of the guide rail, i.e., the overlapping width of the side ends is defined in such manner that the depth of the guide grooves decreases in the forward direction. Furthermore, the axes of rotation of the roller electrodes are rotated from a vertical plane including the longitudinal welding line so as to cross each other at a small angle in a top view.
The flow and relative position of a blank, in the case where the above-mentioned deforming and welding steps are automatically carried out successively to produce can bodies, are described below. In a deformation system including the roll-forming machine, a preceding deformed cylindrical blank is removed from the machine by a hook of the chain. Deformation of the succeeding blank should be completed by the time the next hook appears in the machine. Therefore, it is necessary to make the distance between the hooks approximately twice as long as the length of the blank. Furthermore, since the feeding direction of a flat blank into the roll-forming machine is perpendicular to the conveying direction (i.e., the direction of movement) of the cylindrical blank, the deforming speed of the blank is twice or more as fast as the removal (movement) speed of the blank from the machine. In a welding system including the guide rail and the roller electrodes, the cylindrical blanks should be fed between the roller electrodes keeping a minimum space between the preceding and succeeding blanks to prevent the blanks from coming into contact with each other and to prevent the roller electrodes from coming into contact with each other. Therefore, the movement speed of the cylindrical blank from the deformation system into the welding system is approximately twice as fast as the welding speed of the roller electrodes. Accordingly, the blank is deformed at a speed four times or more as fast as the welding speed, is suddenly stopped, is conveyed at a suddenly accelerated speed approximately twice as fast as the welding speed, is rapidly decelerated in the welding system, and then is fed out of the welding system at the welding speed. Thus, a thin metal blank having a small rigidity is subjected to the above-mentioned sudden actions. As the production rate in a series production of can bodies is increased, the blank is more strongly subjected to inertial forces, shaking (radial displacement of the center axis) of the cylindrical blank, vibration, and frictional resistance, so that it is difficult to feed the blank into the welding station without undesirable influences.
Hitherto, since the production rate (i.e., the welding speed) of can bodies has been relatively low, the undesirable influences of inertia force, shaking (radial displacement of the center axis), vibration, and frictional resistance have been relatively small. Therefore, it has been possible to weld can bodies automatically by pressing the cylindrical blank with elastically-supported guide plates or rollers to bring both side ends of the blank into contact with the bottoms of the two guide grooves of the guide (Z-shape) rail, respectively. The pressure of the side ends on the bottoms has been regulated so that the side ends are not damaged and the forward movement of the cylindrical blank is not impeded. However, if the production speed is increased to a high speed of more than approximately 40 m/min in a conventional apparatus for automatic electric seam resistance welding of can bodies, the above-mentioned undesirable influences are increased and the vibration may become resonant. As a result, the cylindrical blank shakes and the guide plates are shaken by the blank, so that it is impossible to keep the center axis of the cylindrical blank stable. Therefore, on the one hand, the frictional resistance of the guide grooves against the side ends of the blank is frequently increased, so that damage of the side ends may easily occur and/or displacement in the forwarding direction of one of the side ends may easily occur. On the other hand, it is impossible to ensure sufficient pressure for bringing the side ends into contact with the bottoms, so that a uniform overlapping width of the side ends can not be ensured. Accordingly, it is impossible to make a preset overlapping width less than three times as large as the plate thickness of the blank. Namely, it is very difficult to weld the can body with a constant overlapping width, particularly one being less than three times as large as the blank thickness, when the seam welding is carried out at a high speed of more than 40 m/min.
It is therefore an object of the present invention to provide a method of automatic electric seam resistance welding of can bodies at a high speed of more than 40 m/min.
It is another object of the present invention to provide an improved apparatus for carrying out the above method.